Issue with creating high precision Ngon faces
It's a kind of warning that you are creating a concave quad face.

Ther's an article that explains they should be avoided.

Remember that render engine base their calculations on triangled meshes. Concave quad faces, as non planar quad faces, are not automatically splitted in Blender.

You can see how it works by adding a plane and move the vertex with fixed z till is placed inside the triangle made by the other 3 vertices